Indicates that the total number of optional
clauses, minus this number should be mandatory.

Percentage

75%

Indicates that this percent of the total number of
optional clauses are necessary. The number computed from the percentage
is rounded down and used as the minimum.

Negative percentage

-25%

Indicates that this percent of the total
number of optional clauses can be missing. The number computed from the
percentage is rounded down, before being subtracted from the total to
determine the minimum.

Combination

3<90%

A positive integer, followed by the less-than
symbol, followed by any of the previously mentioned specifiers is a
conditional specification. It indicates that if the number of optional
clauses is equal to (or less than) the integer, they are all required,
but if it’s greater than the integer, the specification applies. In this
example: if there are 1 to 3 clauses they are all required, but for 4 or
more clauses only 90% are required.

Multiple combinations

2<-25% 9<-3

Multiple conditional
specifications can be separated by spaces, each one only being valid for
numbers greater than the one before it. In this example: if there are 1
or 2 clauses both are required, if there are 3-9 clauses all but 25% are
required, and if there are more than 9 clauses, all but three are
required.

NOTE:

When dealing with percentages, negative values can be used to get
different behavior in edge cases. 75% and -25% mean the same thing when
dealing with 4 clauses, but when dealing with 5 clauses 75% means 3 are
required, but -25% means 4 are required.

If the calculations based on the specification determine that no
optional clauses are needed, the usual rules about BooleanQueries still
apply at search time (a BooleanQuery containing no required clauses must
still match at least one optional clause)

No matter what number the calculation arrives at, a value greater than
the number of optional clauses, or a value less than 1 will never be
used. (ie: no matter how low or how high the result of the calculation
result is, the minimum number of required matches will never be lower
than 1 or greater than the number of clauses.